A fundamental problem in all vehicles with drive to multiple wheel contact surfaces is how the driving force is distributed. It is desirable to control the speed of the wheels so that the wheel slip in the longitudinal direction is the same at all wheel contact surfaces, since excessive wheel slip at individual wheel contact surfaces is thereby prevented. The wheel slip is the scaled difference between the speed of the wheel at the wheel contact surface and speed of the ground at the same point. Low tractive force at contact surfaces with low friction is automatically compensated for by increased tractive force at contact surfaces with high friction.
Given similar ground conditions, the coefficients of friction utilized will be approximately the same regardless of the prevailing vertical load at each wheel contact surface. This means that the tractive force is automatically distributed in proportion to the vertical load, which gives optimum efficiency in transmitting force to the ground.
One easy way of producing the desired equality in longitudinal slip is to mechanically connect the driving of all wheel contact surfaces in the driveline. Problems arise, however, when cornering. In this context the ground will move at different speeds at the various wheel contact surfaces. The ground under the outer wheels moves at a higher speed than the corresponding inner wheels since the outer wheels have a greater distance to cover in the same time as the inner wheels.
For the same reason, the ground under the front wheels generally moves at a higher speed than under the rear wheels. In certain articulated vehicles, such as loaders, the drive to the front and rear axle is mechanically linked. The aim is to eliminate the problem of different speeds under the two axles by placing the steering joint midway between the front and rear axle, which when driving with a constant radius of curvature gives the same ground speed under both axles. The rotational speeds are the same due to the mechanically linked drive. With superelevation of the stationary vehicle, however, which is a common working situation for loaders, the two axles will be drawn closer to one another which means that the ground under each axle moves in opposite directions. This results in slipping at the wheel contact surfaces and loads in the driveline that will reduce its service life. There is obviously also the outstanding inner/outer wheel problem.
In the majority of vehicles, such as four-wheel drive cars, trucks with 4×4 and 6×6 drive and articulated transport vehicles, the vehicle concepts are such that it is impossible to alleviate the front/rear axle problem through suitable location of the steering joint.
The problem of also distributing the tractive force when cornering is conventionally solved by dividing the torque in a specific ratio by means of a differential. The rotational speed is then controlled by the speed of the ground at the various contact surfaces and by the wheel slip. The fact that wheel slip is not controllable, however, is something of a disadvantage. If the product of the vertical load and the ground friction does not correspond to the torque ratio in the differential, the wheel slip may increase uncontrollably, the wheels slip and the total tractive force transmitted is limited by the slipping wheel contact surface.
The difficulty of the uncontrolled wheel slip is usually alleviated by various measures for braking the wheel slip, for example by using so-called differential locks. A dog clutch which mechanically locks the differential is the oldest and still perhaps the most common solution. The disadvantage is that the speed differential when cornering manifests itself as wheel slip at the actual wheel contact surfaces. This produces great torque loads, which shortens driveline service lives, increases losses and results in heavy tire wear.
Another solution aimed at limiting the slip in the case of differentials is to use the service brake to increase torque at the slipping wheel contact surface and thereby control the slip. The difference in rotational speed in braking corresponds to the vehicle speed, which can result in certain losses.
If the brake torque is instead applied inside the rotating differential, the difference in rotational speed will correspond to the difference in the curve radius and thereby give rise to smaller losses than in the solution using the service brake. In straight-line driving no losses are sustained since the difference in rotational speed is zero. Various solutions are adopted for solving the problem of controlling the brake torque via the differential. One common solution is based on the fact that the brake torque increases in proportion to the transmitted drive torque, through discs or friction in the gear system. Such solutions will result in braking when cornering, even in the current absence of slipping at a wheel contact surface. This results in losses and wear.
It is usual, especially on passenger cars with 4WD, for the main drive to occur on one axle, the second axle contributing to the drive in the event of a rotational speed difference between the axles. So-called viscous couplings are an example of this type of solution, the torque on the second axle increasing when the difference in rotational speed increases. In another similar solution, the difference in rotational speed builds up a cumulative hydraulic pressure, which actuates a multiple disc clutch, which transmits drive torque to the second axle. If the drive axle slips, a difference in rotational speed is created, which transmits torque to the second axle. This is all very well if the vehicle is running in a straight line. When cornering, a difference in rotational speed occurs between front and rear axles because the front axle travels a greater distance than does the rear axle in the same time period. The coupling between front and rear axle cannot differentiate whether the difference in speed is the result of slipping wheels or of cornering. A load is therefore produced, by way of the driveline coupling, between the front and rear axle. This manifests itself as slipping in opposite directions at the wheel contact surfaces of the two axles, which gives rise to forces, braking forces on the front axle and driving forces on the rear axle. Drive torque forces add to this, which can lead to the rear axle slipping away when cornering, the driver being unable to anticipate such behavior. This is obviously a safety risk.
This is accentuated by the tires used on passenger cars nowadays, which are designed to provide high “cornering stiffness;” that is to say, high lateral force absorption at a small angle of drift. This also results in high longitudinal force absorption in the event of slight wheel slip, which means that high longitudinal force is attained even in the event of moderate wheel slip. However, the capacity to withstand lateral force falls rapidly with increased longitudinal force so that the conditions for uncontrolled skidding are soon reached.